The present invention generally relates to intravascular medical devices. More specifically, the present invention relates to medical devices suitable for intravascular ionizing radiation therapy.
Intravascular ionizing radiation therapy is being used increasingly to treat vascular disease. For example, the administration of ionizing radiation has been proposed as both a primary and a secondary therapy for treatment of vascular stenosis (a vascular restriction or narrowing). Clinical studies show that ionizing radiation may be used to inhibit or prevent restenosis after angioplasty.
Vascular restrictions often vary in shape and size, depending on the extent and nature of the disease, in addition to the size and type of vessel affected. In cross-section, the stenotic tissues forming the vascular restrictions often vary in thickness. Such vascular restrictions with varying thickness may require different amounts of radiation exposure, depending on the thickness of the stenotic material and the relative position of the radiation source.
To address this issue, U.S. Pat. No. 6,033,357 to Ciezki et al. propose the use of a radiation delivery device having a window defined by an attenuator for directing radiation emitted from a radiation source. The attenuator alters the radiation exposure pattern such that compensation may be made for any irregular shape of the stenosis or eccentric positioning of the radiation source. In use, a first intravascular ultrasound (IVUS) catheter is inserted into the vascular system to determine the configuration of the vessel wall and the shape of the stenosis. Based on this information, the configuration of the attenuator section is selected to deliver the desired radiation dose profile. The first IVUS catheter is then withdrawn and the radiation delivery system is inserted into the vascular system. A second IVUS catheter is inserted into the radiation delivery system to orient the attenuator such that the window is adjacent the area to receive the most amount of radiation. The second IVUS catheter is then removed from the delivery system and a radioactive wire is inserted into the delivery system until the radioactive portion is positioned within the treatment area. After sufficient time is allowed to emit the proposed dosage, the radiation source is removed from the delivery system.
The Ciezki et al. device inherently relies on maintaining the desired position of the delivery system between withdrawal of the second IVUS catheter and insertion of the radioactive wire. Any difference in position between these two steps will inevitably result in certain portions of the treatment area receiving more or less radiation than intended. Furthermore, if it is necessary to treat other areas of the vasculature, the individual imaging and delivery steps must be repeated in sequence for each area of the vasculature to be treated. Such numerous steps (repositioning, inserting, removing, etc.) complicate the procedure and consume significant operating room/lab time. In addition, the Ciezki et al. device requires many different attenuators to be stocked in a variety of different sizes, shapes, densities and configurations to address different clinical circumstances. Accordingly, it is desirable to provide a radiation delivery system utilizing IVUS technology that is not susceptible to procedural complexities as with the Ciezki et al. system.
The present invention addresses these shortcomings by providing a radiation delivery system that fully integrates intravascular ultrasound (IVUS) technology. The radiation system includes a catheter having a distal head. A fixed or removable radiation source is disposed in or adjacent to the distal head. The distal head includes a radiation shield having a window and an ultrasonic transducer. The ultrasonic transducer facilitates placement of the radiation shield window, such that only a portion of the treatment site is exposed to radiation.
Specifically, the ultrasonic transducer provides a signal indicative of relative position, tissue geometry and/or tissue characteristics which may be utilized to determine the appropriate placement of the radiation shield window. Placement of the radiation shield window affects the dose administered to different portions of the treatment site. Thus, the dose may be varied to target different areas of the treatment site with the desired radiation dose.
The radiation delivery system may include a drive means coupled to the distal head to facilitate rotation thereof. The distal head may rotate at a constant velocity or at a variable velocity. For example, the distal head may rotate at a velocity which varies as a function of distance from the vascular wall and/or as a function of stenotic thickness.
The radiation delivery system may include a retractable sheath having a distally disposed radiation shield positioned over the radiation source. The retractable sheath may be used to shield radiation during intravascular navigating and positioning of the delivery system to avoid undesired radiation exposure.
The present invention also provides a method of treating a vascular site with ionizing radiation utilizing a radiation system substantially as described above. The catheter is first introduced into the vascular system of the patient and advanced until the distal head is disposed adjacent the treatment site. The ultrasonic transducer is then activated to generate data indicative of relative position, tissue geometry and/or tissue characteristics at the treatment site. The radiation shield window is then moved as a function of the data to selectively expose the treatment site to ionizing radiation. If a rotating shield is used, the velocity of rotation may be varied as a function of the data, (e.g., distance from the vascular wall and/or stenotic thickness).